Known methods of producing a thin sulfide film using a vacuum film-forming apparatus include a multi-source deposition method (referred to as "MSD method" hereinafter), a chemical vapor deposition method (referred to as "CVD method" hereinafter), an electron beam deposition method, and a sputtering method. Known film-forming apparatuses include a multi-source deposition apparatus (referred to as "MSD apparatus" hereinafter), a chemical vapor deposition apparatus (referred to as "CVD apparatus" hereinafter), an electron beam deposition apparatus, a sputtering apparatus, and the like.
A thin sulfide film can be readily formed wherein the amount of sulfur in the film is a value smaller than that of the stoichiometric composition. In the CVD method or the sputtering method, a film is formed by introducing a gaseous sulfur compound, e.g., hydrogen sulfide gas or the like, into a vacuum deposition vessel from the outside thereof such that the sulfur loss is decreased. A sputtering method using sulfur as a sputtering gas has also been proposed (refer to Japanese Patent Laid-Open No. 61-213370).
In the electron beam deposition method, although a sintered sulfide is sometimes evaporated without any treatment, co-deposition (referred to as "electron beam co-deposition method" hereinafter) is generally effected in which a crucible, separate from the electron beam deposition source, is provided within the vacuum deposition vessel as the sulfur source in order to prevent a sulfur loss. This separate crucible is at least partially filled with sulfur, and is heated during the electron beam deposition operation to thereby supply excessive sulfur to the substrate to be coated. A reactive deposition method, in which a sulfur compound such as hydrogen sulfide or the like is introduced into a vacuum deposition vessel, can also be employed for the same purpose.
In the MSD method, a plurality of deposition sources, which are contained in a vacuum deposition vessel, are at least partially filled with the respective deposition materials, and the deposition materials are evaporated by separately heating the deposition sources, to form a film on a substrate by the chemical bonding of the vapors of these deposition materials. The thin sulfide film formed by the MSD method has better crystallinity than that of the films formed by other methods. When the thin sulfide film is used in a thin film electroluminescence element, the element can be driven at a low voltage and has a significantly high luminance (refer to Japanese Patent Laid-Open No. 62-160694).
However, the thin sulfide film producing method, such as the CVD method or the sputtering method, in which a gaseous sulfur compound is introduced into a vacuum deposition vessel has a problem with respect to interference with the film functions, where the interference is caused by the non-sulfur component of the gaseous sulfur compound. Since the sulfur compound introduced and the decomposition products thereof frequently have high toxicity, a recovery apparatus is generally required, thereby complicating the whole production apparatus.
The sputtering method which uses sulfur as the sputtering gas has the following problems: the ions of the thin film formed are damaged, and thus a good film cannot be obtained. Since a sputtering target must be cooled in order to prevent damage to the target, the sulfur can easily adhere to the target, and the reproducibility deteriorates due to a change in the composition of the target. In addition, when sulfur vapors are simply introduced into the vacuum deposition vessel, the efficiency of use of the sulfur deteriorates due to the diffusion of the sulfur vapors over the entire volume of the vacuum deposition vessel. Since the sulfur nozzle is formed in the wall surface of the vacuum deposition vessel, the sulfur nozzle cannot be easily heated, and as a result some of the sulfur adheres to the nozzle, thereby deteriorating the controllability. Since the nozzle is at a relatively low temperature, sulfur can form an eight-member ring which has low reactivity, and the efficiency of use of the sulfur thus deteriorates.
When elemental sulfur or a simple sulfur substance is evaporated from a source thereof within the vacuum deposition vessel, as in the conventional electron beam deposition method or the conventional MSD method, the amount of sulfur evaporated can readily vary with any variations in the heat radiated from the other deposition sources, since there are large differences between the vapor pressure of sulfur and the vapor pressures of the other commonly employed deposition materials.
For example, when a thin film serving as a light emitting layer of a thin film electroluminescence element and composed of ZnS:Mn, CaS:Eu, SrS:Ce, or the like, is formed by the evaporation of sulfur within the vacuum deposition vessel, the sulfur must be evaporated in an amount substantially greater than that of the other deposition materials, since sulfur has a lower probability of adhesion to a substrate than many other deposition materials such as Zn, Mn, Ca, Eu, Sr, Ce, and the like. As a result, the level of the molten sulfur in the crucible changes significantly, and the amount of the sulfur evaporated thus changes. Since it is thus necessary to frequently supply sulfur to the crucible in the vacuum deposition vessel, the operation of the apparatus requires considerable time, because the pressure in the vacuum deposition vessel must be raised to atmospheric pressure each time sulfur is to be supplied and then lowered to the vacuum conditions after the sulfur content in the crucible has been increased. In addition, since sulfur is evaporated at a relatively low temperature of 300.degree. C. or less and has a higher vapor pressure than those of most other deposition materials, the amount of the sulfur being evaporated can be changed by the effect of the heat generated from the high temperature deposition sources of the other deposition materials, thereby deteriorating the reproducibility of the film formation.